Field of the Invention
This invention relates generally to a free-machining steel article and to a method for making same. More particularly, the invention relates to elongated product forms such as wire, rod, bar, band, and strip made from a substantially lead-free, free-machining, powder metallurgy steel.
Description of Related Art
Small, precision-machined parts for the watch, automotive, and other industries are made from steel wire that has been cold drawn and then straightened. Good machinability is required for manufacturing such parts and is usually obtained by including in the structure of the steel one or more additives to enhance the machinability property. Pb, S, and Se are among the most common additions to steel for enhancing machinability. However, the addition of Pb has some safety issues. Therefore, a lead-free machining steel with the same or better machinability than leaded steel is desired.
U.S. Pat. No. 8,282,701 B2 and U.S. Pat. No. 8,795,584 B2, describe a lead-free, free-machining steel article for use in the production of high quality, precision parts and a process for making such an article. The microstructure of the steel described in those patents is fine grained and has a fine and uniform distribution of manganese sulfides (MnS). As described in those patents, the microstructure is obtained by using a controlled alloy chemistry, gas atomization to produce alloy powder, followed by hot consolidation of the alloy powder to form a powder compact. A billet is prepared from the powder compact which is then hot worked and cold finished to make wire and bar products for machining into small precision parts.
The alloy and method described in U.S. Pat. No. 8,282,701 B2 and U.S. Pat. No. 8,795,584 B2 has been used to provide small diameter wire and bar with acceptable machining characteristics for making small, precision parts. However, it has been determined in practice that better machinability is needed for the machining of very small precision parts. Accordingly, it is an object of the current invention to provide small diameter wire and bar products that have a finer and more homogenous distribution of carbides to thereby enhance machinability and processability of the wire or bar beyond what was previously achievable with the material made in accordance with U.S. Pat. No. 8,282,701 B2 and U.S. Pat. No. 8,795,584 B2. An additional object of the present invention is to provide a process for making very small diameter wire and bar, including a heat treatment that, in combination with the modified chemistry, will readily provide the desired microstructure in order to obtain the enhanced combination of properties described above. The entire disclosures of U.S. Pat. No. 8,282,701 B2 and U.S. Pat. No. 8,795,584 B2 are hereby incorporated herein by reference.